1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display (LCD), and more particularly, to a method for manufacturing an LCD having a thin film transistor (TFT) as a switching element.
2. Discussion of the Related Art
In an LCD having TFTs as switching elements, the TFTs are integrated so as to drive and control each pixel. As shown in FIG. 1, in a conventional LCD having a TFT array, almost rectangular pixel electrodes 12 are arranged in rows and columns on a substrate 11. Gate lines 13 are formed along the rows and data lines 14 are formed along the columns.
FIG. 2A is a plan view showing a part of LCD elements in the LCD having a TFT array. As shown in FIG. 2A, a gate electrode 23 is formed on the substrate 11 and a plurality of data lines 14 perpendicularly crossing gate lines 13 are formed in parallel with each other. TFTs are formed near each crossing point of the gate line 13 and the data line 14.
FIG. 2B is a cross-sectional view taken along line I--I in FIG. 2A. As shown in FIG. 2B, a gate electrode 23 made of Ta is formed on the substrate 11. A gate insulating layer 21 made of SiN.sub.x is formed on the whole surface of the substrate including the gate electrode 23, and a semiconductor layer 22 made of a-Si is formed on the gate insulating layer 21. An ohmic contact layer 33 made of n.sup.+ a-Si is formed on the semiconductor layer. Source and drain electrodes 24, 34 made of Mo are formed on the ohmic contact layer 33. Then, an organic layer 10 made of organic material is formed as a passivation layer.
The organic material has a smoother surface profile than inorganic materials. Therefore, when the organic material is deposited on the surface of the LCD, which has an uneven surface profile due to the multi-layered elements, the stepped surface can be flattened. Thus, defects such as non-uniform orientation condition or misalignment of liquid crystal molecules due to the stepped surface can be reduced. Also, a higher aperture ratio can be obtained by increasing the area of pixel electrodes.
Next, an inorganic insulating layer 15 made of SiO.sub.2 or SiN.sub.x is formed on the organic insulating layer 10. A contact hole is formed and finally, a pixel electrode 12 made of a transparent conductive material such as ITO is formed.
As described above, the passivation layer of the conventional LCD has a layered structure having organic and inorganic insulating layers 10, 15. The inorganic insulating layer 15 is formed over the organic passivation layer in order to assist the adhesion property of the ITO layer onto the passivation layer. This is because the organic passivation layer does not provide a good adhesion to the ITO layer.
There are two ways of forming the organic and inorganic insulating layers. One is to sequentially deposit the organic and inorganic insulating layers and pattern them simultaneously. The other is to deposit and pattern the organic insulating layer first and then form the inorganic insulating layer.
First, the former has the following problem. The process of patterning the organic and inorganic insulating layers includes utilizing an organic solution, such as a mixture of N-Methyl-Pyrolidone ("NMP"), alcohol, and amine, to remove the photoresist. Then the insulating layer may swell or expand because such an organic solution can penetrate through the interface between the organic passivation layer and the inorganic insulating layer.
Second, the latter also has a problem in that two patterning steps are needed for forming the insulating layer. In addition, the drain electrode may be disconnected because the difference in thermal expansion coefficients between the organic and inorganic insulating layers may produce cracks in the contact hole area.